We propose basic photochemical studies relevant to the synthesis of 1 alpha,25-dihydroxyvitamin D3, phototherapy of psoriasis, phototherapy of neonatal jaundice, and skin photosensitization by medicines and pesticides. Our long range goal is a molecular level understanding of photochemical and photobiological phenomena which will have predictive value for the future. Our immediate objectives are (1) an efficient in vitro synthesis of 1 alpha,25-dihydroxyvitamin D3, (2) a better understanding of the molecular details of steroid photochemistry, (3) utilization of photochemical methods in synthesis, (4) utilization of reactive intermediates in synthesis, (5) synthesis of reactive molecules of theoretical interest, and (6) a better understanding of heterocyclic photochemistry. Our unique low temperature (1.3 degrees K-80 degrees K) photochemical techniques offer special advantages in these studies.